Developments concerning applications of nanomaterials are continuously increasing. The reports of scientific commissions (for example such as the AFSSET 2006 report in France entitled “Rapport AFSSET—les nanomatériaux: effets sur la santé de l'homme et sur l'environnement—juillet 2006” [AFSETT report—Nanomaterials: effects on human health and on the environment—July 2006]) show that in the next ten years the economy will be strongly affected by the production of nanomaterials, given that they involve numerous socioeconomic domains such as energy, materials for energy, medicine, electronics, cosmetics, and composite materials, to mention only a few examples. These nanomaterials are going to appear in a great variety of forms that depend closely on the technologies concerned. According to M. C. Roco [M. C. Roco, “International perspective on government nanotechnology funding in 2005”, J. Nanopart. Res. 7(6), pp. 707-712, 2005], they may be classified into four major domains, as follows:
1) passive nanostructures involving both dispersed and agglomerated nanostructures (aerosols, colloids) and materials incorporating nanostructures (coatings, nanoparticle reinforcement of composites, nanostructured metals, polymers, ceramics, etc. . . . );
2) active nanostructures that comprise those that are bioactive and that have medical (or health) effects (molecules that are carried and taken to identified targets, biodevices, . . . ) and those that have physicochemical activity (three-dimensional transistors, amplifiers, actuators, adaptive structures, . . . );
3) nanosystems such as guided assemblies, three-dimensional networks, and hierarchical architectures, robotics, . . . ; and
4) molecular nanosystems relating to molecular and atomic devices, emerging functions, . . . .
The present growth of such nanomaterials in all fields of human activity leads legitimately to questions being raised on the health and environmental consequences. In a recent article in the journal “Technology Review” [www.technologyreview.fr/nano-tech/?id=196], mention is made of health problems associated with everyday use of compositions (e.g. household cleaners) even though goods of this type have doubtless obtained all of the health authorizations that are presently required before being launched on the market. Similarly, carbon nanoparticles have been accused of being responsible for pathology of the optic nerve in mice [G. Oberdörster, E. Oberdörster, J. Oberdörster, “Concepts of nanoparticle dose metric and response metric”, in Environ Health Perspect. 2007, June, 115(6):A290]—(G. Oberdörster, University of Rochester). Other studies have revealed problems associated with the presence of fullerenes (E. Oberdörster, Duke University) or of carbon nanotubes [Chuff-Wing Lam, John T. James, Richard McCluskey, and Robert L. Hunter, Toxicological Sciences 77, pp. 126-134 (2004)] (C. W. Lam, NASA, Houston). Other substances are regularly being added to the list. That has naturally drawn attention to the validity of the criteria used for validating products and their individual ingredients. At present, a case-by-case rule is used since there is no methodology that is adapted to validating nanomaterials, thereby raising a fundamental problem.
Furthermore, the appearance of nanoparticles in the plasmas used in the fabrication methods of microelectronic industries lead to irremediable defects in the devices being made. The reject rate may exceed 50% in certain sectors. That situation also applies to white rooms, i.e. to the immediate environment of machines, and also of operators, where the measurement systems are capable of measuring powders in suspension in air providing powder size is greater than or equal to 0.5 micrometers (μm). Thus, needs in terms of in situ detection and measurement in gas are of great importance.
Most of the methods implemented at present are based on the interaction between a light beam and powders. They can be found in a variety of versions for in situ measurements such as light scattering (including lidars) and laser-induced incandescence. Other versions require samples to be collected and put into suspension in an aqueous solution in order to perform ex situ measurements. For all methods enabling characterization to be performed in situ, it is necessary to have optical access into the systems involved (reactors, etc.) and that is far from being available in most industrial reactors. In addition, scattering cross-sections become very small for powders of nanometric size and the intensity of the scattered light is completely drowned in noise. Consequently, in order to perform measurements that are reliable on powders of nanometric size, it is necessary to seek novel methods that are non-intrusive and that do not require optical access and that do not require samples to be taken.
Numerous businesses are to be found on the market making use of those optical techniques, sometimes in association with systems for charging particles in order to facilitate manipulation thereof. By way of example, those techniques enable particles to be segregated by size. Nevertheless, it is not possible under such circumstances to answer questions associated with the concentration of powders at the places from which they were taken. Among such businesses, mention may be made of some that are very active in this market:                GRIMM (Germany)        MALVERN (Great Britain)        TSI (USA)        NANOSIGHT (Great Britain)        NANEUM (Great Britain)        CILAS (France).        
The technologies developed by those businesses make use of light being scattered (or diffracted) by particles in suspension in a liquid solution in which they have previously been immersed. That technique, together with various variants thereof, such as dynamic light scattering, nevertheless presents limits associated with the scattering cross-section when acting on particles in the nanometric range of sizes. With scattering, the scattered intensity is proportional to rp6 where rp is the radius of the particles in the powder. Scattering is consequently much more sensitive to the presence of aggregates.
In order to improve performance, another method has been developed by TSI that is based on the mobility of particles. That method, known as scanning mobility particle sizer (SMPS) enables particles to be separated by charge and by electric mobility in order to classify them by size. The particles are initially charged by a corona discharge method. SMPS technology makes it possible to measure particles in aerosols with concentrations of 107 particles per cubic centimeter (cm3).
In order to make that technique more sensitive to particles in the nanometric size range, Coulter, followed by TSI, make use of the effect of water vapor condensing on the surface of nanoparticles in order to make them more “visible” in laser light scattering. Nevertheless, it is legitimate under such circumstances to ask the following question: is it the “clad” size of the particle that is being measured or its real size?
As a result of research undertaken in the GREMI laboratory over more than fifteen years concerning the formation of nanoparticles in low pressure cold plasmas, it has been shown that it is possible to make use of the modifications in the properties of the discharge induced by the powders to provide detector means. On this topic, reference may be made to the publication by L. Boufendi et al. entitled “Detection of particles of less than 5 nm in diameter formed in an argon-silane capacitively coupled radiofrequency discharge” (Applied Physics Letters—Vol. 79, No. 26—Dec. 24, 2001).
Since then, several research teams in the world have made use of that method in their work. No research work has used that approach for in situ and real time measurement of the size and the concentration of particles of powder that are formed and that remain trapped in levitation in a plasma.